1. Field of the Invention
The present invention relates to a structure of a solenoid air valve used in a blood pressure monitor or the like.
2. Description of the Background Art
Blood pressure is measured based on information on artery obtained by placing an air bag around human body (brachium, wrist) and inflating the same with air to apply pressure to the human body. When inflated by an air pump, the air bag is closed so as to avoid air leakage. At the time of measurement, deflation control is performed. When the measurement is finished, the inflated air bag needs to be deflated, in order to release the human body from constraint. A solenoid air valve is often used in such a case. A conventional air valve structure is disclosed in Japanese Patent Laying-Open No. 09-135817 (hereinafter, referred to as document 1) and Japanese Patent Laying-Open No. 2000-097364 (hereinafter, referred to as document 2) described below.
According to the air valve structure disclosed in document 1, a nipple is pressed against and brought in contact with a packing portion provided at a tip end of a movable plunger by prescribed elastic force by a spring or the like, thereby controlling air flow rate. In addition, the air valve structure disclosed in document 2 is driven by a moving core or a moving coil, in which a packing is brought in contact with a nipple so as to control a pressure of air that flows, and a viscous member is provided between the moving core or the moving coil and a solenoid fixing member so as to smoothly control air flow rate. In this manner, in the air valve according to the conventional structure, the nipple serving as an air outlet is pressed and closed by sufficient driving force so as to avoid air leakage during inflation. While measurement is being performed and when measurement is finished, the packing is moved in order to control deflation and to release air pressure.
In recent years, in order to improve portability also of a blood pressure monitor, reduction in size of a main unit of the blood pressure monitor is demanded, which means that the air valve contained therein should also be made smaller. The conventional structure, however, suffers from the following problems. The packing for sufficiently covering the nipple portion is necessary for confining air. When the packing is brought in direct contact with the moving core and the moving core is made thinner for size reduction, magnetic reluctance is increased and driving force necessary for pressing the packing is lowered, which results in failure in achieving sufficient sealing and reduction in size.
FIGS. 5 and 6 show a cross-sectional structure of a conventional solenoid air valve 200. A frame 201 has a frame main body 202 and a frame cover 203. Frame main body 202 accommodates a bobbin 241. A coil body 260 is accommodated between an outer circumferential surface of bobbin 241 and an inner circumferential surface of frame main body 202. In addition, in bobbin 241, a fixed core 210 and a moving core 220 serving as a slider are coaxially arranged, and a coil spring 230 applying force in a direction separating fixed core 21 and moving core 220 is accommodated therebetween. In the fixed core, an air passage 211 is provided. A convex nipple (air outlet) 212 is provided at a position of the fixed core opposed to moving core 220, and a rubber packing 250 is embedded in a position of moving core 220 opposed to nipple 212.
In a normal state (a state in which a current is not fed to coil body 260), as shown in FIG. 6, rubber packing 250 of moving core 220 and nipple 212 of fixed core 210 are separated from each other by means of coil spring 230, whereby air can flow out through air passage 211. On the other hand, in a closed state, as shown in FIG. 7, the current is fed to coil body 260, so that magnetic flux is generated by excitation of coil body 260 and moving core 220 is attracted to and brought in contact with fixed core 210. In addition, nipple 212 is pressed by rubber packing 250 to close air passage 211.
Here, reduction in size of air valve 200 structured as above is considered. As shown in FIG. 8, it is possible to make smaller an outer diameter of frame 201, as well as to make smaller an outer diameter φ1 of fixed core 210 and moving core 220 for ensuring a volume of coil body 260. Taking into account the air flow rate, however, an inner diameter of nipple 212 cannot be made smaller, or a diameter of rubber packing 250 for closing nipple 212 cannot be changed either. Therefore, as shown in FIG. 9, an area of magnetic pole of moving core 220 around rubber packing 250 is made smaller. In such a case, magnetic flux M is concentrated in a narrow area around rubber packing 250, which results in increase in magnetic reluctance, lower efficiency of a magnetic circuit, and lowering in the driving force. Consequently, attractive force of fixed core 210 to be exerted on moving core 220 may extremely be lowered, and leakage of air from nipple 212 is caused. Meanwhile, in order to obtain attractive force equivalent to that in the conventional example in the more compact structure shown in FIG. 8, the current should be increased, which results in increase in a current value.
In addition, as shown in FIG. 10, an end surface portion of moving core 220 opposite to fixed core 210 is covered by bobbin 241 made of resin composed of a non-magnetic material. As a gap L1 comparable to a thickness of the resin is present between frame cover 203 and moving core 220 implementing the magnetic circuit, magnetic reluctance at gap L1 is increased, which may become a factor to lower efficiency of the magnetic circuit.